1. Field of the Invention
This invention relates to a method of coating a substrate in a fluidized bed. More particularly, this invention relates to a method of coating a substrate which may have a high surface area in a fluidized bed using coating source material which is fed into the bed in condensed form.
2. Description of the Related Art
Processes to form films on substrates, e.g., chemical vapor deposition (CVD) processes, conventionally transport the coating source material to the reactor containing the substrate to be coated in a gaseous or vapor, i.e., noncondensed, state comprising one or more of the following constituents: (a) a precursor for the material to be deposited; (b) reducing, oxidizing, or otherwise reacting gases; and (c) inert carrier gases used for chemical transport and/or heat and mass transfer. As these materials enter the reactor, which is kept at a high temperature, reactions occur and products are formed which deposit on the substrate surface.
While the different components of the coating system are conventionally all introduced into the coating reactor as gases or vapors, if the components are solids or liquids at room temperature, they must first be vaporized and then transported into the reactor as vapors. To control the rate of feed to the reactor, the vapor pressure and the flow rate have to be carefully controlled.
Such careful control of vapor pressure, in turn requires vaporizers with sophisticated thermal controls, and heated feed lines from the vaporizers to the reactor. Typically thermal baths are used to maintain the temperature of the vaporizer within a fraction of a degree. In addition, calibration runs are needed, because the saturation of the carrier gas and the control of flow are also sources of error. When very precise compositions are required, the control equipment and the need for constant calibrations result in low productivity, low reliability, and high equipment and production costs.
When relatively unstable molecules, such as organo-metallics, are used as coating precursors, a further limitation occurs. Many of these molecules start vaporizing and decomposing or polymerizing within a narrow temperature range. To avoid decomposition or polymerization in the evaporator, the temperature is kept as low as possible, but this results in a very low vapor pressure which, in turn, results in very low deposition rates.
In any of these coating systems, the use of coating source materials in gaseous or vapor (non-condensed) form is particularly troublesome when attempting to coat high surface area materials, e.g., very small particles, because of the difficulty of feeding large quantities of coating source material into the reactor in the gaseous or vapor state.
It would, however, be advantageous to provide an economical film forming process that could avoid or minimize the need for sophisticated, expensive, and difficult to maintain evaporators, and the limitations in operating conditions attendant with the use of such evaporators; and which would be capable of quickly and thoroughly coating large and/or complex surface area objects.